This invention relates to a paste for filling holes in ceramic substrates and a method for using the paste.
Ceramic circuit boards or substrates are often used in the manufacture of electronic equipment due to their ruggedness and resistance to environmental extremes. For example, ceramic circuit boards are commonly used in the manufacture of hybrid circuits, i.e., electronic circuits mounted on "thick film" ceramic substrates. These hybrid circuits may then be employed in a wide range of devices, such as electronic test equipment, computers, and electronic equipment for aircraft, just to name a few. Several other uses for ceramic hybrid circuits are shown and described in the October 1993 issue of Semiconductor International at page 56 in an article entitled "Will Hybrid Circuits Survive?", the disclosure of which is hereby incorporated by reference.
Most hybrid circuits of the kind described above typically comprise a ceramic substrate to which are mounted a variety of raw integrated circuit chips, i.e., integrated circuits that have not been encapsulated in a protective plastic package. The various integrated circuit chips are electrically connected to one another by a plurality of conductive metallic traces (printed circuits) deposited directly on the substrate. In most cases, the substrate also includes passive electronic components, such as resistors and capacitors, required to make the circuit functional. The passive circuit elements are usually manufactured directly on the ceramic substrate by any number of well-known processes. Once all of the electronic components are in place, they are enclosed by a cover which provides mechanical protection and hermetically seals the components from the external environment.
While it is possible to fabricate a functional hybrid circuit by placing all of the electronic components and conductive traces on the front side of the ceramic substrate, it is often necessary to include additional conductive traces on the back side of the substrate, particularly if the circuit is complex. If that is the case, the conductive traces on the back side of the substrate are usually connected to the appropriate conductive traces on the front side of the substrate by electrically conductive vias or holes. Basically, an electrically conductive hole is formed by drilling a small hole through the substrate at the appropriate location. The hole is then made electrically conductive by depositing a thin layer of metal on the walls of the hole by any one of a number of well-known processes. In this manner it is possible to provide the additional electrical connections required for complex circuits. However, even if such additional conductive traces are not required, it is common to deposit a thin layer of metal on the back side of the substrate to act as a ground plane. If so, the appropriate ground points on the conductive traces on the front side of the substrate are connected to the ground plane on the back side of the substrate by electrically conductive holes.
While the use of such electrically conductive holes is convenient and has solved many problems, they often prevent the formation of a true hermetic seal (i.e., a seal impervious to both liquids and gases) between the electronic components on the front surface of the substrate and the external environment. Specifically, if the holes themselves are not hermetically sealed, then the environmental immunity, thus reliability, of the entire hybrid circuit is compromised. Consequently, several methods have been developed in an effort to close the holes and orovide the desired hermetic seal.
One method for closing the holes has been to cement a small epoxy-coated fiberglass disk or "manhole cover" over the hole. Unfortunately, however, the epoxy resins that are commonly used to cement the disk or "manhole cover" to the substrate are water permeable, thus do not provide a true hermetic seal. Also, the process of placing and securing a disk over each individual hole in the substrate is tedious and time consuming. Moreover, since the disks are relatively large, it may be difficult to seal holes that are close together.
Other methods fill the holes with epoxy resins or, alternatively, silicone elastomers. Such methods generally include the steps of placing a screen or template over the substrate to mask the entire substrate except for the holes and then force the fill material into the exposed holes by a suitable means, such as a squeegee. While such methods eliminate the tedious chore of placing a ceramic disk over each individual hole, they are no panacea, and suffer from their own disadvantages. For example, it is difficult to achieve a reliable seal and even if a reliable seal is achieved, it is usually not hermetic. Moreover, the coefficient of thermal expansion of the fill material is usually quite different from the coefficient of thermal expansion of the substrate, thus creating the possibility of hole leakage when the circuit is exposed to extreme temperatures. Another disadvantage is that small variations in the composition of the fill material tend to greatly affect the reliability of the seal. For example, even minor variations that exist between different lots of the same material have caused problems. Obviously, such extreme sensitivity to minor variations in material composition poses significant problems.
In an attempt to overcome some of the foregoing problems, sintered metals, such as gold, have been tried as substitutes for the epoxy resin and silicone elastomer fillers described above. Unfortunately, however, using sintered metals to seal the holes creates a variety of different problems. For example, several filling and sintering steps are commonly required to completely fill the hole, which increases production costs. Also, the resulting seal is not hermetic, so little is gained by using the sintered metal fill material.
Consequently, there remains a need to provide a true hermetic seal for holes in ceramic substrates. The sealing method should be relatively easy to perform and require a minimum number of steps. The material used to fill the hole should be inexpensive, easy to handle, and inert. Moreover, the quality of the seal should not be overly sensitive to minor, but expected, variations in material composition. Finally, the sealing material should be universally applicable with respect to holes which are coated with a metallic composition or uncoated.